Magnetic Structure and Properties of CrAs.

Selte, Kari; Kjekshus, Arne; Jamison, Warren E.; Andresen, Arne F.; Engebretsen, Jan E.; Ehrenberg, L.

doi:10.3891/acta.chem.scand.25-1703

Cited by 87 publications

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“…The magnetic transition of the first order into a double-helical state occurs at T N ∼ 265 K. This transition is accompanied by a large magnetostriction, and a lowering of the crystal structure symmetry has not been reported [3,4]. The propagation vector is incommensurate with 0.354 · 2πc * , where c * is a unit vector along the c axis, and magnetic moments of ∼ 1.7µ B /Cr lie in the ab plane [5,6]. The application of pressure drastically suppresses T N , and the HM phase disappears above a critical pressure of P c ∼ 0.7 GPa [1,2,7].…”

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confidence: 95%

“…Because the nuclear spin of 75 As is I = 3/2, and because all the As sites in the crystal are equivalent to each other, three transitions were expected for a commensurate magnetic structure, whereas complicated spectra composed of six peaks were obtained at both 5 and 100 K. Each peak being not well separated suggests the presence of a spatial modulation of the internal field at the As sites. In fact, previous neutron scattering measurements have shown that the magnetic structure is incommensurate [5,6]. Be-…”

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confidence: 98%

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Detection of an Unconventional Superconducting Phase in the Vicinity of the Strong First-Order Magnetic Transition in CrAs UsingAs75-Nuclear Quadrupole Resonance

et al. 2015

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Pressure-induced superconductivity was recently discovered in the binary helimagnet CrAs. We report the results of measurements of nuclear quadrupole resonance for CrAs under pressure. In the vicinity of the critical pressure Pc between the helimagnetic (HM) and paramagnetic (PM) phases, a phase separation is observed. The large internal field remaining in the phase-separated HM state indicates that the HM phase disappears through a strong first-order transition. This indicates the absence of a quantum critical point in CrAs; however, the nuclear spin-lattice relaxation rate 1/T1 reveals that substantial magnetic fluctuations are present in the PM state. The absence of a coherence effect in 1/T1 in the superconducting state provides evidence that CrAs is the first Cr-based unconventional superconductor. In a conventional Bardeen-Cooper-Schrieffer (BCS) superconductor, electrons form Cooper pairs through negative interactions via an electron-phonon coupling. This gives a sign-uniform s-wave character to the superconducting (SC) gap symmetry. If positive interactions work electron pairs to induce superconductivity, unconventional superconductivity with a sign-changing order parameter is realized beyond the well-established simple s wave. This fascinating form of superconductivity has been discovered in specific materials, such as heavy fermion systems, high-T c cuprates, organic systems, ruthenate, and recent Fe-based superconductors. A common feature of these materials is that superconductivity appears due to the instability of some degree of freedom, which is mainly induced on the verge of a magnetically ordered phase.Very recently, Wu et al. and Kotegawa et al. independently discovered pressure-induced superconductivity in CrAs in the vicinity of the helimagnetic (HM) phase [1,2]. This is the first example of superconductivity found in a Cr-based magnetic system. CrAs has a MnPtype orthorhombic crystal structure with a P nma space group. This structure possesses a space-inversion symmetry, but it is locally missing at the Cr and As sites, and both of these atoms form a zigzag chain along the a-axis. The magnetic transition of the first order into a double-helical state occurs at T N ∼ 265 K. This transition is accompanied by a large magnetostriction, and a lowering of the crystal structure symmetry has not been reported [3,4]. The propagation vector is incommensurate with 0.354 · 2πc * , where c * is a unit vector along the c axis, and magnetic moments of ∼ 1.7µ B /Cr lie in the ab plane [5,6]. The application of pressure drastically suppresses T N , and the HM phase disappears above a critical pressure of P c ∼ 0.7 GPa [1,2,7]. Superconductivity appears together with the suppression of the HM phase, showing a maximum SC transition temperature of T c ∼ 2.2 K at ∼ 1.0 GPa, after which T c decreases gradually with increasing pressure. The pressure-temperature phase diagram of CrAs with a first-order magnetic transition is reminiscent of those of some pressure-induced Fe-based superconductors, such as SrFe 2 As 2 ...

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Detection of an Unconventional Superconducting Phase in the Vicinity of the Strong First-Order Magnetic Transition in CrAs UsingAs75-Nuclear Quadrupole Resonance

et al. 2015

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“…13 Thus, CrAs shows antiferromagnetic behavior. In this work, we restrict ourselves to collinear magnetic structures, as the energy differences due to noncollinear structures are expected to be small.…”

Section: Bulk Crasmentioning

confidence: 99%

“…13 In this structure, both Mn and As atoms are sitting on the 4c Wyckoff positions: ͑x , 1 4 , z͒, ͑−x , 3 4 ,−z͒,…”

Section: Bulk Crasmentioning

confidence: 99%

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Stable structure and magnetic state of ultrathin CrAs films on GaAs(001): A density functional theory study

2010

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Density functional theory calculations using the pseudopotential-plane-wave approach are employed to investigate the structural and magnetic properties of epitaxial CrAs thin films on GaAs͑001͒. Motivated by recent reports of ferromagnetism in this system, we compare zinc-blende CrAs films ͑continuing the lattice structure of the GaAs substrate͒ and CrAs films with a bulklike orthorhombic structure epitaxially matched to three units of the GaAs͑001͒ lattice. We find that even for very thin films with three Cr layers the bulklike crystal structure is energetically more favorable than zinc-blende CrAs on GaAs͑001͒. CrAs films with orthorhombic structure, even if under epitaxial strain, preserve the antiferromagnetic order of CrAs bulk. In the light of our calculations, it appears likely that the magnetic hysteresis loop measured in ultrathin CrAs/GaAs͑001͒ films originates from uncompensated antiferromagnetic moments near the CrAs/GaAs interface. In conclusion, our results do not support earlier proposals that thick CrAs films could be employed as perfectly matched spin-injection electrode on GaAs. I. MOTIVATION AND BACKGROUNDSpin injection from a ferromagnet into a semiconductor is a major prerequisite for semiconductor spintronics. In recent years it has become clear that the efficiency of spin injection not only depends on the degree of spin polarization in the ferromagnet but the ferromagnet-semiconductor interface also plays a crucial role. Using electrodes fabricated from conventional ferromagnetic ͑FM͒ materials, such as Fe or Co, only a limited amount of spin-polarized current could be injected into GaAs. 1,2 As possible reasons, the conductivity mismatch in the metal and the ͑doped͒ semiconductor has been discussed 3 but the unsatisfactory results could also be due to technological problems in fabricating high-quality interfaces.As an alternative, Akinaga et al. 4 suggested to grow CrAs epitaxially, in the zinc-blende ͑ZB͒ structure, on GaAs. As advantages over conventional ferromagnetic electrodes, it has been pointed out that the use of a zinc-blende CrAs layer on GaAs not only avoids ͑possibly detrimental͒ interface states 5 but in addition zinc-blende CrAs is a half-metallic ferromagnet according to DFT calculations, i.e., it offers the highest possible spin polarization of 100%. By growing a superlattice of alternating CrAs and GaAs layers, Akinaga et al. managed to fabricate heterostructures showing ferromagnetic behavior at room temperature. Following this work, several research groups studied epitaxial CrAs films on GaAs, 6-11 and confirmed the ferromagnetic hysteresis of the samples. The saturation magnetization was found to correspond to about 2 B up to 3 B per Cr atom. However, the remanent magnetization was typically much lower, less than 10% of the saturation magnetization. Transmission electron microscopy ͑TEM͒ images of the samples demonstrated pseudomorphic growth of CrAs on GaAs, and most researchers claimed the formation of zinc-blende CrAs. However, Etgens et al., 12 using in situ refl...

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Double exchange in CrxMn1−xas compounds1)

Wohl

Berg

Bärner

1980

Phys. Stat. Sol. (a)

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The magnetic moment M and the specific resistivity ϱ of CrxMn1−xAs polycrystals are measured for temperatures between 5 and 300 K and for eight different compositions. Structures in the ϱ(T) and M(T) curves define characteristic temperatures which are used to construct a magnetic phase diagram. The large number of magnetic phases between ferromagnetic MnAs and antiferromagnetic CrAs are explained on the basis of two competing superexchange–double exchange magnetic couplings.

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Magnetic Structure and Properties of CrAs.

Cited by 87 publications

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Detection of an Unconventional Superconducting Phase in the Vicinity of the Strong First-Order Magnetic Transition in CrAs UsingAs75-Nuclear Quadrupole Resonance

Detection of an Unconventional Superconducting Phase in the Vicinity of the Strong First-Order Magnetic Transition in CrAs UsingAs75-Nuclear Quadrupole Resonance

Stable structure and magnetic state of ultrathin CrAs films on GaAs(001): A density functional theory study

Double exchange in CrxMn1−xas compounds1)

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